This invention relates to the field of modulating a carrier frequency with a modulating signal. More specifically, the invention relates to the modulation of a carrier frequency of a resonant circuit used in radio frequency tagging.
A resonant circuit is one in which the values of circuit resistance, R, capacitance, C, and inductance, L, are chosen such that the reactance of the resonant circuit is a minimum at a resonant frequency.
In amplitude modulated radio reception, the antenna circuit is tuned to resonate with the carrier frequency of the particular radio frequency circuit. This tuning to resonance or near resonance of the RF antenna circuit provides a means for discriminating against the many other carrier frequencies used by other broadcasting stations, thereby allowing the listener to choose one station without interference from the others broadcasting simultaneously.
Information content is impressed on the carrier via a mixing scheme known as modulation in which a nonlinear clement combines the carrier with the frequencies containing the information to yield a sum and difference frequency signal. This mixing process takes place at the broadcast station. The information is subsequently retrieved by an amplitude modulated radio receiver which separates the carrier via rectification and filtering leaving only the information signal, a method generally referred to as demodulation. Other types of modulating and demodulating circuits are well known, e.g., circuits using frequency modulation, and pulse modulation.
In prior Radio Frequency (RF) tagging art, a resonant circuit is disposed on a thin insulating dielectric substrate to form a tag for use in electronic article detection (EAS) schemes. Generally, the coil of the resonant circuit consists of a closed loop of a conducting element which has a certain value of resistance and inductance. A capacitive element which forms part of this closed loop consists of two separate areas of thin metal conducting film disposed on opposite sides of the dielectric. The tag is attached to articles to be protected from theft. An RF signal at or near the resonant frequency of the resonant circuit is emitted from a base station. When the tag is in the RF field, the tag""s absorption can lead to a change in the tank circuit current of the base station and a power dip in a receiving coil. Either one of these two effects can be used to sense the presence of the tag and hence the item to which it is attached. Thus, an alarm can be made to sound when either of these effects are sensed by a pickup coil or by an amplifier, indicating improper removal of an item. To deactivate the tag, a relatively high RF power pulse can be applied at the counter at which the point-of-sale of the item takes place. This high power acts to short the capacitor or burn out a weak portion of the coil. In either case, the circuit is no longer resonant and will not respond to the RF interrogation from the base station. In that case, the customer who has made a legitimate purchase at the point-of-sale counter can pass through the interrogation-sensing gate without setting off an alarm. It is clear from this description that these tags, once deactivated, arc not reusable. In addition, in the configuration just described, the tags are capable of only conveying one bit of information. Thus, they cannot give any information regarding the item""s identification and are useful only for anti-theft applications. This kind of tag is normally classified as a single bit tag.
Some RF tags consist of a resonant coil or a double sided coil containing two thin film capacitors with the plate of each capacitor on opposite sides of the dielectric. Such tags can be used for source tagging and have an initial frequency that is different from the frequency used at the retail establishment for theft protection. For example, in U.S. Pat. No. 5,081,445 (assigned to Checkpoint), the tag is designated as being in a deactivated state until the first capacitor is shorted by means of a high power RF pulse at the then resonant frequency. Disabling the capacitor shifts the resonant frequency of the RF circuit to the store interrogation frequency. A second deactivation pulse is used to disable the second capacitor at the point-of-sale when payment is received for the item to which the tag is attached. At this stage, the tag is no longer usable and has been permanently destroyed.
Some additional art discloses two or more frequencies that can be obtained on a RF coil tag by altering the capacitance of the circuit. In one case, a strong DC electric field is applied to change the effective dielectric constant of the capacitor. Thus, the circuit has two resonant frequencies depending on the value of the applied electric field. Due to the ferroelectric hysteresis, the tag can be deactivated by the application of a DC field. However, it can also be reactivated and hence re-used by applying a DC field of opposite polarity (U.S. Pat. No. 5,257,009, assigned to Sensormatic). In an earlier embodiment, a set of capacitors connected in parallel attached to an inductance have been described in U.S. Pat. No. 5,111,186 assigned to Sensormatic in which each dielectric of the set of capacitors varies in thickness. In this manner, a series of resonant frequencies can be obtained by applying different voltages (electric fields). Each of the capacitors then changes capacitance at a different electric field (voltage) levels depending on the thickness of the dielectric. There can be some concern regarding the high voltages required for creating the change in the dielectric. Also, using this apparatus, the remanent state of ferroelectric tends not to be very stable for long periods of time. Additional concern relates to the dielectrics, which are also piezoelectric materials which have properties quite sensitive to stress.
U.S. Pat. No. 5,218,189, assigned to Checkpoint, provides an array of series capacitors connected in parallel with an inductor. Here, the resonance can be altered by selectively shorting one or more of the capacitors, thereby changing the resonant frequency of the resulting circuit. A frequency code can thereby be established by disabling or burning out selective capacitors at the time of interrogation, those capacitors becoming disabled which at the time of manufacture of the tag were xe2x80x9cdimpledxe2x80x9d. The disadvantage is that the item or person is subject to high r.f. fields during interrogation. Also, the range of frequencies that needs to be scanned is necessarily large. This makes detection difficult since the requirement to scan a large band of frequencies puts a strong demand on the flat response of the detector circuit.
U.S. Pat. No. 4,745,401 describes an embodiment for a reusable tag. It is comprised of two ferromagnetic elements, one soft (low coercivity) and one hard (high coercivity) both physically covering a portion of an R.F. coil. The ferromagnetic element with high coercivity can be magnetized to apply a bias field to the soft material to put the latter into saturation. In that state, the R.F. field generates very small hysteresis losses leading to a relatively high Q of the tag circuit. On the other hand, when the hard magnet is demagnetized, the RF Field results in hysteresis losses in the soft material which lowers the Q of the circuit. This change in Q can be used to determine whether a tag is active or has been deactivated. While this constitutes a reusable tag, the change in Q tends to be small and thereby some what more difficult to distinguish from other effects that attenuate the absorption.
In U.S. Pat. No. 3,500,373 an apparatus is described for interrogating and sensing the presence of a RF resonant tag. Here the interrogating frequency is swept around a center frequency. In general, there is very little radiation emitted except when the tag is present in the field of tile emitter. Thus, when there is no tag in the antenna field, very little energy is lost from the antenna circuit. When the swept frequency coincides with the resonant frequency of an active tag, energy is absorbed and a sensing circuit detects a drop in voltage level in the interrogating antenna oscillator circuit. The tag absorption occurs twice with every complete sweep cycle resulting in a negative dip in the oscillator circuit. The negative dip causes pulse modulation which is filtered, demodulated and amplified to cause an alarm to be activated, indicating theft of an item. Thus, the basic detection is achieved by varying the interrogation carrier frequency to match the resonance of a tag whose center frequencies span a range depending on the type or make of tag.
As already stated, the above prior art senses changes in the resonant state of a tag, either by changing its Q or changing the frequency of the tag circuit. The cited embodiments are detected by way of sensing the change in the magnitude of the tag absorption at the resonant frequency or a change in the Q of the tag circuit. However, since these tags generally operate in the MHz regime, they are easily shielded so that the signal both to and from the tag is readily attenuated. Since sensing the tag relies strongly on the absorption or the carrier and the frequency spectrum of the pulse occurring during the frequency sweep, pulse detection has difficulties under a variety of conditions. Sweeping introduces harmonics or higher frequencies so that the signal to noise is degraded. Therefore, the demodulation of the carrier containing the pulse as a result of the tag in the field of the swept carrier necessarily contains more noise than a carrier modulated by a quasi cw sine wave. For some tags, it may also be hard to distinguish a change in Q from a change in position or orientation of the tag relative to the RF field direction. Therefore, these prior art tags can produce weak signals that are difficult to discriminate at the base station (transceiver).
Many of the prior art RF tags are limited to one bit of information and are not reusable. Many of the reusable tags in the prior art require the use of very strong fields.
An object of this invention is an improved radio frequency (RF) tag transponder.
An object of this invention is an improved, reusable RF tag with one or more bits of information.
Another object of this invention is an improved, reusable RF tag that creates a dependable and easy to discriminate signal at the base station.
An object of this invention is a tag with a reusable resonant circuit that is capable of modulating a carrier signal with one or more discrete and highly detectable modulating frequencies of an applied energy field.
The present invention is a transponder apparatus that uses a resonant circuit with one or more variable circuit components. The resonant circuit has a resonance frequency at or near the frequency of a radio frequency (RF) carrier.
A system and a method can include a base station that communicates with the transponder by using an RF carrier.
The base station interrogates the resonant circuit by using an RF carrier signal with a frequency at which the resonant circuit resonates and a modulating signal that cause one or more components of the resonant circuit to vary at a component frequency there by varying the resonance frequency of the resonance circuit. As the resonance frequency of the resonance circuit varies, the RF carrier signal is modulated by the modulating signal.
The values of one or more of the components (resistance, inductance, and capacitance) or the resonant circuit are varied by one or more remote modulating signals. The modulating signals are transmitted from a location (or locations) remote from the resonant circuit. The modulating signals are external energy fields that vary continually without interruption over a period of time in magnitude and/or frequency at a modulating signal frequency. For example, the modulating signal can be an acoustic field or an audio frequency electromagnetic field.
In many of the preferred embodiments, the varying component value(s) of the resonant circuit vary due to a mechanical change of the component. The mechanical change of the component (at the component frequency), and therefore the change in value of the component, varies most when the frequency of the force produced by the modulating signal(s) is equal to one of the mechanical resonances of the varying component(s) of the resonant circuit.
The modulated RF carrier is demodulated, typically at the base station, by using a receiver (demodulator) tuned to demodulate the carrier to obtain the encoded information (like the modulating signal) from the carrier.
Various novel preferred embodiments of these variable components of the resonant circuit are disclosed, including capacitors with vibrating plates (physically constrained in various ways, e.g. as a cantilever, sliding plate, etc.); inductors with variable permeability and/or mutual inductance; and variable resistance.
In a preferred embodiment, the resonant circuit is used in a radio frequency (RF) tag. Various preferred embodiments of the RF tag include means for establishing a code of one or more bits by introducing one or more variable components of the RLC circuit. Each component has mechanical resonance or a state that responds more at one of the modulating signal frequencies than at other frequencies. An RF tag with multiple bits is made by using components of the RLC circuit with different states on a single RF tag.
In a preferred embodiment, the system and tag transponders are used for anti-theft protection as well as item identification. In addition, the system includes means for interrogating and detecting the device to retrieve the information carried by the tag.